448 ZOOLOGY SECT. 



The heart is completely divided into two halves a right and a 

 left between which there is no aperture of communication. Each 

 half consists of an auricle and a ventricle, opening into one another 

 by a wide opening, guarded by a valve composed of three 

 membranous cusps on the right side, two on the left. The right 

 ventricle gives off the pulmonary artery ; the left gives off the 

 .single aortic arch, which passes over to the left side, turning round 

 the left bronchus in order to run backwards as the dorsal aorta : it 

 therefore represents the left aortic arch of Reptiles. The blood is 

 warm. The red blood corpuscles are non-nucleated and usually 

 circular. 



The two cerebral hemispheres, in all but the Monotremes and 

 Marsupials, are connected together by a band of transverse fibres 

 -the corpus callosum not represented in the lower Vertebrates. 

 'The dorsal part of the mid-brain is divided into four optic lobes- 

 the corpora quadrigemina. On the ventral side of the hind-brain 

 is a transverse band of fibres the pons Varolii by which the 

 lateral portions of the cerebellum are connected together. 



The ureters, except in the Prototheria, open into the bladder. 



Mammals are all, with the exception of the Monotremes, 

 viviparous. The foetus is nourished before birth from the blood- 

 system of the parent through a special development of the foetal 

 membranes and the lining membrane of the uterus, termed the 

 placenta. After birth the young Mammal is nourished for a longer 

 or shorter time by the milk, or secretion of the mammary glands of 

 the parent. 



Sub-class I, Prototheria. 



Mammals in which the mammary glands are devoid of teats ; 

 the oviducts are distinct throughout, and there is a cloaca into 

 which the ureters and urinary bladder open separately. In the 



centra of the vertebras the epiphyses are absent or imperfectly 

 developed ; the bones of the skull early coalesce by the oblitera- 

 tion of the sutures ; there is a large coracoid articulating with 

 the sternum, and a T"- sna P e d episternum, and there is a pair of 



epipubic (marsupial) bones. A corpus callosum is absent. The 

 ova are meroblastic, and are discharged in an early stage of their 

 development, enclosed in a tough shell. 



This sub-class comprises a single living order, the Monotrcmata, 

 including the Duck-Bill or Platypus (Ornithorhynclius) and the 

 Spiny Anteater (Echidna), together probably with an imperfectly 

 known extinct, Secondary and early Tertiary order, the Multi- 

 tuber cidata. 



Sub-class II. Theria. 



Mammals in which the mammary glands are provided with 

 teats ; the oviducts are united in a longer or shorter part of their 

 extent, and there is no cloaca; the ureters open into the base 



